1. Field of the Invention
The invention relates to a coolant or refrigerant turbo-compressor for compressing water vapor under vacuum conditions. The compressor has a plurality of impeller vanes and preferably is of a radial type.
2. Description of the Related Art
In order to protect the environment from the effects of some currently used coolants or refrigerants, it has become very important to develop and employ new refrigerants that are environmentally safe. In this context, water is a noteworthy alternative, but has previously not often been used as a coolant or refrigerant. The physical process of using water as a refrigerant or coolant has long been known. For example, as early as 1755, the Scotsman W. Cullen used a vacuum pump to vaporize water, thereby realizing a mechanical means of generating cooling or providing refrigeration.
For decades it has also been known to use water as a coolant or refrigerant in connection with absorption refrigeration plants and steam jet refrigeration plants. Similarly, it has long been known to use vapor compression apparatus in which water vapor is compressed and thereby raised to a higher energy level, for the purpose of generating heating steam, predominantly using turbo-compressors of a radial construction type. However, these machines are not economically applicable to refrigeration apparatus using water as a working medium, because the temperature ranges of the two different applications are substantially different. For example, the compressor intake or suction temperatures in vapor compression apparatus are in the range of approximately 80.degree. to 120.degree. C. On the other hand, refrigeration plants using water as a refrigerant require an intake temperature in a range between 0.degree. and 50.degree. C.
While such temperatures are also achieved in a steam jet refrigeration plant, the energy efficiency achieved is much lower than that of refrigeration plants using mechanical compression. The density of water vapor in refrigeration plants is up to 3 powers of ten less than the density involved in the vapor compression process or that involved in the compression of classical refrigerants. Due to the extremely low density of the water vapor, it is necessary to pump extremely large volume flows of the refrigerant through the refrigerating apparatus. Furthermore, it is necessary to provide compression ratios (.pi.) of .pi..apprxeq.5 in order to carry out the method.
While positive displacement compressors, such as known screw-type compressors for example, can develop the required compression ratio, such compressors are very limited in their maximum delivered volume flow and furthermore are considerably too expensive. On the other hand, single-stage kinetic or flow-type compressors, for example turbo-compressors of the radial type, cannot achieve the compression ratio required for use in a refrigeration apparatus. Furthermore, such compressors are quite expensive because they generally are designed for compressing gases or vapors having a considerably higher density, for example air, and therefore have been designed and constructed to be driven with a comparatively much higher specific drive power.
The vanes or blades of known radial compressor impellers are typically connected to a supporting rotor disk by welding or riveting, whereby the rivets are inserted through the vane or milled onto the vane. These known connecting methods cause problems, especially in compressors for compressing water vapor, wherein the impeller must have a large number of impeller vanes and each vane must be quite wide. In this case, it becomes increasingly difficult to attach the vanes to a supporting rotor disk in the typical manner, because the flow cross-sectional area remaining between the vanes becomes ever smaller or closed, the supporting disk becomes weakened, for example by rivet holes, or the grain structure is altered due to welding.
Highly mechanically loaded radial compressor impellets, i.e. so-called limit output impellers, are predominantly cast of steel or duralumin high strength aluminum alloy, forged and then machined by milling. Thus, the entire limit output impeller is a single integral piece. However, such one-piece cast, forged, and milled impellers are complicated and expensive to manufacture and suffer other disadvantages as well.
In order to achieve a smooth intake, it has been proved effective to bend the intake portion of the vanes in the circumferential direction or to use an intake impeller, which is predominantly a cast impeller. Such an intake impeller forms the intake portion or inlet zone of the impeller vanes. Such intake impellers have a relatively small diameter as compared to the outer diameter of the main impeller itself, and are therefore subjected to comparatively light mechanical loads. The attached or following radial vane, i.e. a radial fiber vane, is superior in material strength to all the other vanes. For this reason it is used in high compression ratio applications in which a high static pressure increase is required, in an apparatus having the smallest possible dimensions and without a particularly high efficiency. In such apparatus, circumferential velocities of up to 600 m/s are carried out.
It is already known to use fiber reinforced composite materials for the impellers of ventilators and for the vanes or blades of axial ventilators and ship's propellers. However, such embodiments using fiber reinforced composite material blades or vanes are only suitable for circumferential velocities up to a maximum of 100 m/s and are thus absolutely not suitable for limit output impellers.
Special turbo-compressors are required for compressing water vapor in the temperature and power range pertinent to refrigeration or cooling technology. Furthermore, such special turbo-compressors must be able to provide a high volume flow rate at a high compression ratio, while operating at a high energy efficiency. The price of such special turbo-compressors must be competitive when compared to typical prior art refrigerant compressors. Finally, it must be considered that very high centrifugal forces arise in radial-type turbo-compressors for high-power water vapor refrigeration apparatus due to the extraordinarily high circumferential velocities, in the range of 500 m/s for example. The centrifugal forces are the major load acting on the impeller, because the forces that must be applied or transmitted to the flow medium are comparatively small.